A ridge waveguide has a lower cutoff frequency than a rectangular waveguide, thereby exhibiting broadband transmission characteristics (patent literature 1). Since the ridge waveguide has favorable transmission characteristics also in the low frequency band, the ridge waveguide can be realized in a smaller size than the rectangular waveguide at the same design frequency. Adopting the ridge waveguide as a transmission line of a radio frequency circuit has an advantage of realizing the radio frequency circuit in a physically smaller space at the same design frequency.